Nanomaterials, such as silicon nanowires (SiNWs), carbon nanotubes (CNTs), and graphene, have gained much attention for use in electrical biosensors due to their nanoscopic and electrical properties. See, for example, Cui et al., Science 2001, 293, 1289; Patolsky et al. Protoc. 2006, 1, 1711; Bradley et al. Nano Lett. 2004, 4, 253; Chen et al. J. Am. Chem. Soc. 2004, 126, 1563; Star et al., Nano Lett. 2004, 4, 1587; Tang et al., Nano Lett. 2006, 6, 1632; Lu et al., ACS Nano 2008, 2, 1825; and Mao et al., Adv. Mater. 2010, 22, 3521.
For instance, SiNWs and CNTs can be integrated into field-effect transistors (FETs) to detect small amounts of target biomolecules with high sensitivity and selectivity by measuring electrical disturbances induced by the binding of these biomolecules to the surface of the nanostructure. See, for example, Woolley et al., Nat. Biotechnol. 2000, 18, 760; and Zheng et al., Nat. Biotechnol. 2005, 23, 1294. The detection of biomarker proteins with high sensitivity and selectivity is vital for the early diagnosis of many diseases such as cancer and HIV. For this purpose, carbon-based nanomaterials like CNTs and graphene have become attractive for fabricating highly sensitive FET-based biosensors. See, for example, Du et al., Anal. Chem. 2010, 82, 2989; Jung et al., Angew. Chem. Int. Ed. 2010, 49, 5708; Lu et al., Angew. Chem. Int. Ed. 2009, 48, 4785; Ohno et al., Nano Lett. 2009, 9, 3318; and Shan et al., Anal. Chem. 2009, 81, 2378. In particular, the use of graphene in FET-based biosensors is becoming more and more appealing not only due to its unique properties such as higher 2-D electrical conductivity, superb mechanical flexibility, large surface area, and high chemical and thermal stability, but also due to its ability to overcome the limitations of CNTs such as variations in electrical properties of CNT-based devices and the limited surface area of CNTs. See, for example, Avouris, Nano Lett. 2010, 10, 4285; Novoselov et al., Science 2004, 306, 666; Kim et al., Nature 2009, 457, 706; Li et al., Science 2009, 324, 1312; Novoselov et al., Nature 2005, 438, 197; Zhang et al., Nature 2005, 438, 201; Geim et al., Nat. Mater. 2007, 6, 183; Jiao et al., Nature 2009, 458, 877; and Du et al., Nat. Nanotechnol. 2008, 3, 491.
Nevertheless, there have been only a few reports on developing graphene FET-based biosensors, and their potential as biosensors has not been fully explored. See, for example, Dong et al., Adv. Mater. 2010, 22, 1649. It would therefore be desirable to develop nanoscopic graphene-based biosensors that are simple in device structures, small in size, and allow label-free detection and real-time monitoring of biomarkers, all of which are useful criteria for biosensors.